Add image description text for the audio splice rendering image

[chrisn]

See #312

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[marcoscaceres]

Having multiple files seems sub optimal, as the description will likely fall out of sync. Can we instead just throw the relevant information into a <details> element? Also, the information should be written to be suitable for all audiences, not specifically for someone using assistive technology (it seems somewhat excessive in its description).

[tidoust]

That looks good to me, thanks for addressing the issue!

[tidoust]

FWIW, the spec-prod action used not to support automatic publication of local HTML assets linked from a spec, and the absolute URL was thus needed (the file does not exist under www.w3.org/TR yet). But that's no longer the case, that feature got added to spec-prod 3 years ago (and I of course forgot to update the URL in MSE accordingly!).

[tidoust]

Hadn't seen @marcoscaceres's comment before I reviewed. This PR has the advantage of using a consistent scheme and approach for both images in the spec, but I don't disagree that the description (in both cases) seems excessive and would better be integrated within the main spec. Up to you :)

[marcoscaceres]

Let's move what we can into the main document. Having these kind of linked descriptions don't make a lot of sense and can quickly get out of sync.